1. Field of the Invention
The present invention relates to a demultiplexer connected to an antenna for use in communication equipment, the demultiplexer having greatly improved characteristics and greatly reduced size, and further, the present invention relates to a communication device including such a demultiplexer.
2. Description of the Related Art
In recent years, many filters have been used in a small-sized communication device such as a portable telephone or the like. An example of the above-mentioned filters is a demultiplexer as shown in FIG. 13. The demultiplexer is provided with a transmission filter and a reception filter to isolate a signal in a transmission band (824 MHz to 849 MHz) and a signal in a reception band (869 MHz to 894 MHz) from each other. The reception band is set to be higher than the transmission band.
It has been more urgently needed to reduce the sizes, the heights, and the weights of small-sized communication devices. Also, such requirements have also been demanded of the above-described demultiplexers. Moreover, in the small-sized communication devices, higher communication frequencies have been used with increases in the volume of communication information and data, such as image data, streaming video data or the like.
Thus, surface acoustic wave filters having surface acoustic wave resonators combined with each other into a ladder configuration and piezoelectric filters having piezoelectric thin-film resonators combined with each other into a ladder configuration have been used for the above-described demultiplexers. These filters can be reduced in size, height, and weight, which has been required with increasing of the communication frequencies.
As shown in FIG. 15, such a known demultiplexer as described above includes a phase-matching circuit 52 connected to an antenna terminal 51, a transmission filter 53 and a reception filter 54 each connected to the phase matching circuit 52, a transmission terminal (Tx) 55 connected to the transmission filter 53, and a reception terminal (Rx) 56 connected to the reception filter 54.
Referring to the demultiplexer, according to Japanese Unexamined Patent Application Publication No. 10-270976 (Patent Document 1), to improve the isolation characteristic caused in the direction extending from the transmission side to the reception side, the grounding elements for the parallel arm resonators 53a of the transmission filter 53 are intentionally separated. As a result, the resonators 53a are grounded via plural ground positions 53b. The grounding elements for the parallel arm resonator 54a of the reception filter 54 are intentionally separated. Thus, the resonators 54a are grounded via plural ground positions 54b. Thus, the isolation characteristic is improved.
Referring to FIG. 15, inductance components are caused between the ground terminal of the parallel arm resonators 53a and the ground positions 53b and also between the ground terminal of the parallel arm resonators 54a and the ground positions 54b, due to wires or through holes which function as connection lines.
Moreover, according to Japanese Unexamined Patent Application Publication No. 2001-53577 (Patent Document 2), to improve out-of-band attenuation in a higher frequency band compared to a pass band, grounding elements for the parallel arm resonators of a transmission filter are electrically connected to each other via a common element, and then, the parallel arm resonators are grounded. Grounding elements for the parallel arm resonator of a reception filter are electrically connected to each other via a common element. Then, the parallel arm resonators are grounded. Moreover, the grounding elements for the parallel arm resonators of the transmission filter and the grounding elements for the parallel arm resonators of the reception filter are electrically connected to each other via a common element. Thus, the out-of-band attenuation is improved.
However, according to the known technique described in Patent Document 1, the ground electrode area for constituting the ground positions has a limitation on the realization of a satisfactory isolation characteristic, since the market requires the reduction in size of demultiplexers.
In particular, according to the known technique described in Patent Document 1, the number of ground terminals for grounding filter-chips must be larger than that of the parallel arm piezoelectric elements combined in the ladder configuration. Accordingly, the number of the ground terminals, and the number of ground electrode patterns extending from the ground terminals to the filter chips are increased. Thus, the size of the package becomes large, and the lamination-number of the multi-layer structure is increased. Thus, problems occur in that the height and the size of the demultiplexer are increased.
Moreover, according to the known technique described in Patent Document 2, the grounding element for the parallel arm resonators of the transmission filter and that for the parallel arm resonator of the reception filter are electrically connected to each other via a common element. Problematically, this deteriorates the isolation characteristic.
As seen in the above-description, according to the known demultiplexers, it has been difficult to realize both of the sufficient isolation characteristic and the reduction in size.